Understanding email bounce messages
What the most common delivery failure codes mean.
When an email can't be delivered, you get a message back explaining why. Look for a code and a short reason.
| Code / text | Meaning | What to do |
|---|---|---|
| 550 User unknown / No such user | The address doesn't exist | Check the spelling of the address |
| 552 / Mailbox full / Quota exceeded | The recipient's mailbox is full | They must free space; try later |
| 550 / 554 Rejected / Spam / Blocked | The receiving server refused your message | Set up SPF, DKIM and DMARC; avoid spam-like content |
| 421 / 450 Try again later | Temporary problem at the receiving side | Usually retried automatically; wait |
| Relay not permitted / Authentication required | Your app isn't signing in to the outgoing server | Turn on SMTP authentication (settings) |
Send the full bounce message in a ticket to Professional Emails if you need help reading it.